How many iPhones did Apple sell last quarter?

The Street is looking for a falloff in sales. The amateurs are looking for a blowout.

I expect Apple (AAPL) to report two important iPhone statistics next week.

The first, the number of iPhone 4S units sold this coming weekend, the company won’t know until after midnight Sunday. The other, the number of iPhones sold last quarter, it already has in hand.

In this post, I’m going to look at the second number, which my two teams of analysts — professional and amateur — have been tracking since the end of June.

The two groups rarely agree, but the discrepancy this quarter is unusually large.

The estimates of the 28 professional analysts who have submitted their numbers to Fortune.com range from a high of 23 million from FBN’s Shebly Seyrafi to a low of 16.92 million from Goldman Sachs’ Bill Shope. The average among this group is 19.82 million iPhones, a bit below the record 20.34 million the company sold in the June quarter.

The average among our 16 amateurs, by contrast, is 22.91 million, which would set a new record. Among this group, the estimates range from a low of 19.52 million from Apple Finance Board‘s Alexis Cabot to a high of 26.8 million from Asymco‘s Horace Dediu.

We’ll find out who was closest to the mark when Apple reports its earnings after the markets close next Tuesday, Oct. 18.